JOHN WILLIAMSON SUBSEA OPTICAL SYSTEMS
cable, ready for service in February 2021, was reported to be the first long-haul 12-fibre pair system to use the technology. 2Africa and Equiano are among the cable systems planning to use SDM.
customers. “In the old days we would have shared capacity using OTN switches, but they do not scale to the kinds of capacity in a cable landing station today,” he comments. “So all-optical sharing is key – that is what spectrum sharing is about.” Ciena sets some store by improved operational and management capabilities. “Streaming telemetry and analytics, coupled with software control and automation, will allow operators to ensure their submarine network assets are operating in an optimal manner by continually adapting to ever- changing conditions,” says Hollebecque. “Reactive capabilities allow operators to intelligently and automatically reroute traffic around inevitable faults. Proactive capabilities, like preventive maintenance, allow operators to address outages before they occur, such as rerouting traffic around a faulty transmitter that’s showing signs of impending failure.” For Luchesini, the evolution of CapEx in new wet plant is of primary importance. “Many analysts point that capacity demand growth requires much more new buildouts than (are) currently planned in the next four years,” he says. “If this is true, it will further drive demand for coherent, high capacity and cost-optimised systems.” Among other things, Clesca puts some emphasis on fibre innovation, and mentions smaller Outer Diameter (OD) fibre to pack more fibre cores in the existing submarine cable design and structure. He references research to develop multi-core optical fibres to overcome the limited space that constrains an increase in the number of fibre cores in submarine cable and repeater structures. “When such multi-core fibres are ready for manufacturing and deployment, they will require the development of a new ecosystem of components and sub- systems – for example Multiple-Input Multiple-Output (MIMO) optical amplifiers,” he concludes.
less power consumption, more operational flexibility, and density.
On a system level what can be achieved with advanced solutions is indicated by the MAREA transatlantic cable system. Entering service in April 2018, it was designed with a goal of 20Tbps per fibre pair but the transponders maxed out at 18Tbps. Applying the fourth and sixth generations of Infinera’s Infinite Capacity Engine (ICE) resulted first in 26.2Tbps ‘hero’ capacity and 24Tbps ‘commercial’ capacity, and then 30Tbps hero and 28Tbps commercial. Hero is maximum error-free transmission capacity, commercial is deployable capacity, including margin to account for link degradation.
Another power-related subsea system issue concerns the material used in cable conductors. Clesca notes that some technical papers show that aluminium used as a conductor is less costly compared to copper. “Additionally, aluminium also allows for lower DC resistance at lower cost and reduces the cable voltage drop, which then allows for an increased number of fibre pairs,” he states. “Facebook has claimed that the 2Africa cable system will make use of cables built with aluminium used as a conductor. It will be very interesting to follow the installation and the first years of operation of this cable system”.
In recent years a new generation of Spatial Division Multiplexing (SDM) or High Fiber Count (HFC) wet plants has been introduced. The SDM approach is based on using more fibre pairs in the cable, each operated in a quasi linear regime, transmission-wise. “This design makes a more efficient use of the optical pump power available inside the repeaters,” says Clesca. “And it requires less complex and less expensive line fibre, with a typical effective area ranging from 80 to 110 µm2, instead of 150 µm2 in a MAREA-like design.” Power is a central consideration for SDM. “There is a lot of modelling that says if you apply, for example, half the amp power you get more than half the capacity,” reports Bennett. “So SDM is about running amps at lower power so that you can support more fibre pairs in the cable. You may have slightly less capacity per fibre pair, but you have a lot more fibre pairs, and that means more capacity per cable.” “Combining modern coherent modems with new SDM wet plants brings overall cable capacities to unprecedented levels that are well into the hundreds of Terabits per second,” adds Hollebecque.
TURN ON, TUNE IN OR DROP OUT?
But when does it make sense to retire a subsea route and build a new one? Clesca makes the point that the cost of O&M along the cable lifetime is virtually independent from the number of bits transported per second and that, therefore, the higher the system capacity, the lower the O&M cost per unit of capacity. As such, he observes that cable economic life is defined as the period where annual unit cost of O&M must be lower than the annual unit price of capacity on newer cable systems on the same routes. “The key factor that dictates retirement date is competition from newer cable systems.”
What characteristics and technologies are these newer and future cable systems likely to feature? Bennett is predicting we’ll see more spectrum sharing as a means of making capacity more affordable for individual
Google’s Dunant 6,600km transatlantic
Bertrand Clesca, director, client solutions, Pioneer Consulting
Geoff Bennett, director of solutions & technology, Infinera
Luca Luchesini, senior director, subsea business development, Nokia
Olivier Courtois, director, submarine product management, Alcatel
Virginie Hollebecque, VP & MD , Western Europe and Middle East, Ciena
ISSUE 24 | Q2 2021
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